A laser oscillator of the kind recited above is known from DE 40 02 162 C1, comprising a laser medium which emits radiation of such broad band that the laser oscillator is tunable, i.e. capable of being varied or adjusted by means of a wavelength selective element.
The fundamental structure of tunable laser oscillators comprising a beam broadening means is known, for instance, from DE 29 18 863 C2. These laser oscillators contain a laser medium emitting broadband radiation, for example a dye solution, excimers produced by gas discharge, or solid body materials. A dispersive element is disposed inside the resonator for tuning of the wavelength. This tuning element (wavelength selective element), for instance, may be in the form of a grating or a dispersing prism assembly. Other suitable wavelength selective elements are an etalon, a Fabry-Perot interferometer, or birefringent crystals.
DD 228 117 A1 discloses a resonator arrangement for a tunable laser, comprising a single reflecting structural member of such design that it produces good spectral filtering at high wavelength selectivity and narrowbandedness. Continuous tuning of the wavelength is achieved by pressure variation. The laser beam is broadened by a system of prisms.
A paper by R. Konig, S. Mory, and A. Rosenfeld published in J. Phys. E.: Sci. Instrum. 20 (1987), at pages 200 to 203, describes a pulsed dye laser with which the beam is widened in the resonator by means of prisms and the wavelength is tuned by rotation of a grating and/or an FP etalon.
A narrowband, tunable laser which comprises a means for decoupling of a reference beam from the laser resonator, including two wavelength selective elements for narrowband tuning of the laser, is described in DE 40 02 162 C1 mentioned initially. The direction of the reference beam depends on the wavelength, and a change in direction of the reference beam is utilized to derive a control signal for control of the angular orientation of a wavelength selective element of the laser. The direction of another partial beam likewise is dependent on the wavelength.
The decoupled laser radiation of the arrangement according to DE 29 18 863 C2 contains both narrowband radiation whose spectral distribution essentially is determined by the beam broadening device and the wavelength selective element as well as relatively broadband radiation whose spectral distribution essentially is determined by the broadband emitting laser medium. This broadband spontaneous radiation is referred to as ASE, for Amplified Spontaneous Emission. The ratio between the energy of the narrowband laser radiation and the energy of the ASE is called the spectral purity of the output radiation of the laser oscillator. The spectral purity of the output radiation of the laser deteriorates, for example, if the laser medium is stimulated in pulsating fashion and the period of the stimulating pulses is of the same order of magnitude as the roundtrip time of the light inside the laser resonator.
The spectral purity of laser radiation also suffers, for example, if the output radiation is intensified further as soon as the ASE and the spectrally pure laser light (in other words the relatively narrowband radiation mentioned above) take different courses in time inside the laser. In general, the ASE is emitted prior to the laser radiation.